The present invention relates to an optical extensometer, and a reference line mark therefor, and more particularly to a mark provided at a reference line in a test piece in order to discern the position of the reference line in case an elongation between the reference lines is measured by using image data obtained by photographing the test piece by a camera.
For example, as an extensometer for measuring, without contacting, an elongation occurred when a tensile load is applied to a test piece by a material testing machine, there has been known a so-called optical extensometer using photograph signals for the test piece by a camera.
In the optical extensometer, in order to discern respective reference line positions on the test piece from the photograph signal by the camera, marks are provided at the respective reference line positions. Normally, the reference line marks are directly printed or drawn on a front surface of the test piece, or a sticker on which the reference line mark is printed is adhered at a pertinent position on the test piece. Also, in order to make a clear difference of contrast in a picture signal from the camera, for example, in case of the sticker, the mark is applied in black on a white ground, or white on a black ground such that the reflectance relative to the back ground is extremely different from each other. And, in case the mark is directly printed on the front surface of the test piece, the mark is provided by a color having reflectance which is different from reflectance of the front surface of the test piece.
Conventionally, as shown in FIG. 5, the reference line marks as described above are located at positions corresponding to the respective reference lines, and include simple straight lines extending in a direction y perpendicular to a deformation (elongation) direction x of a test piece W.
In the optical extensometer as described above, generally, in order to specify the respective reference line positions based on the photograph signal of the test piece from the camera, the following operations are carried out. Each picture element or pixel data is integrated in the direction (y direction in FIG. 5) perpendicular to the deformation (elongation) direction of the test piece to obtain a profile P(x) of a shade in the elongation direction (x direction) of the test piece as shown in FIG. 6. An increasing point and a decreasing point of the profile P(x), i.e. x direction coordinates of intersections between the profile P(x) and a threshold level Th, are calculated to obtain, for example, a center position therebetween as a reference line position. Incidentally, L designates a pitch of the picture element in the figure.
Also, in case of obtaining a resolution greater than that of the picture element or pixel of the camera, an interpolation operation of the aforementioned profile P(x) is required. In order to achieve the first degree in JIS (Japanese Industrial Standards), i.e. less than 3 .mu.m or 1% of the elongation in precision, an interpolation operation with high accuracy is indispensable. Here, in this interpolation operation, practically, it is effective to lower the degree in the approximate formula so as to accelerate computing speed, and in case of approximating by a linear equation which has the fastest computing speed, the increasing point A and the decreasing point B are calculated by the following equations. ##EQU1##
In this case, however, there is a problem that an error in the computation is influenced by a shape of the profile. The error tends to become 0 when a true edge is located on a center of the picture element, and tends to become maximum when the true edge is located away from the center of the picture element with a distance of a half size of the picture element. For example, if the pitch of the picture element is 0.1 mm, there can be seen a phenomenon that the position of the mark as a result of computation is meandered in a cyclic movement of 0.1 mm as shown in FIG. 7. As a result, measured results for the elongation are meandered, and especially, meandering in a proportional elastic area of a material results in deteriorating an accuracy of computing an elastic modulus.
Incidentally, although the shape of the profile greatly depends on adjustments of focus and contrast, if the focus is optimally adjusted and contrast is toned up, a meandering degree is increased on the contrary.
The present invention has been made in view of the aforementioned problems, and an object of the present invention is to provide a reference line mark for an optical extensometer, wherein a reference line position with high accuracy can be always specified by an interpolation computation using a linear equation which is the most advantageous in view of the computing speed, so that an elongation with high accuracy can be measured at high speed.
Another object of the invention is to provide an optical extensometer formed of the reference line mark and a camera for providing photograph signals of the test piece with the mark main body.
Further objects and advantages of the invention will be apparent from the following description of the invention.